Document that we can't use new stdlib symbols until added to type-checkers

https://github.com/python/typeshed/pull/11890#discussion_r1597530831
An unwritten convention (because we've been bitten by it a few times in the past), is to not use newly added symbols from the stdlib stubs until all supported type-checkers (pyright, mypy, pytype??, realistically we're always waiting on mypy release because pyright releases so often) updated their vendored version of typeshed.

This is a suggestion to add this convention to the contributing guideline in a more official way, and to teach contributors about this unobvious gotcha.

Relates to #5835

まず contributing guideline と typeshed PR 11890 の議論、特にリンクされたコメントを読んで、既存の規約を理解してください。新しく追加された stdlib の stub シンボルは、サポート対象の type-checker が vendored typeshed のバージョンを更新するまで待つべきであることを文書化してください。contributors にとってガイダンスが明確になり、関連する guideline が更新されれば完了です。